Multi-piece solid golf ball

ABSTRACT

In a multi-piece solid golf ball comprising a solid core consisting of a center core and an outer core, an inner cover layer and an outer cover layer, the solid core is molded from a rubber composition comprising a base rubber composed of (a) a polybutadiene having a high cis-1,4 content, a minimal 1,2 vinyl content and a viscosity η of up to 600 mPa·s at 25° C. as a 5 wt % toluene solution, being synthesized using a rare-earth catalyst, in combination with (b) another diene rubber, (c) an unsaturated carboxylic acid, (d) an organosulfur compound, (e) an inorganic filler, and (f) an organic peroxide; and the center core has a specific JIS-C hardness on its center and a specific JIS-C hardness on its surface and the certain difference, the outer core is harder than the surface hardness of the center core, the cross-sectional hardness of 1 mm outside from the border between the center core and the outer core is a specific range on JIS-C hardness, the surface of the outer core has a specific JIS-C hardness, the inner cover layer has a specific Shore D hardness; the outer cover layer has a specific Shore D hardness; and the outer cover layer has a lower Shore D hardness than the inner cover layer. This combination of features gives the ball a good, soft feel upon impact and an excellent spin performance that provides increased distance.

CROSS REFERRENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 10/635,603filed on Aug. 7, 2003 continuation-in-part of application Ser. No.10/156,950 filed on May 30, 2002, now U.S. Pat. No. 6,634,961, theentire contents of which are hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multi-piece solid golf ball which hasbeen imparted with a good, soft feel upon impact and an excellent spinperformance that makes it possible to achieve an increased distance.

2. Prior Art

Various improvements are being made in formulating the polybutadieneused as the base rubber in golf balls so as to confer the balls withoutstanding rebound characteristics.

For example, JP-A 62-89750 describes rubber compositions for use as thebase rubber in solid golf balls, which compositions are arrived at byblending a polybutadiene having a Mooney viscosity of 70 to 100 andsynthesized using a nickel or cobalt catalyst with another polybutadienehaving a Mooney viscosity of 30 to 90 and synthesized using a lanthanidecatalyst or polybutadiene having a Mooney viscosity of 20 to 50 andsynthesized using a nickel or cobalt catalyst.

However, further improvements in the materials are required in the aboveart to achieve golf balls endowed with a good, soft feel upon impact andan excellent spin performance that helps increase the distance the balltravels when played.

JP-A 2-268778 describes golf balls molded using a blend composed of apolybutadiene having a Mooney viscosity of less than 50 and synthesizedusing a Group VIII catalyst in combination with a polybutadiene having aMooney viscosity of less than 50 and synthesized with a lanthanidecatalyst. However, golf balls with a good, soft feel upon impact and anexcellent spin performance that helps increase the distance traveled bythe ball cannot be obtained in this way.

The existing art also teaches multi-piece solid golf balls in which anintermediate layer is molded of a low-Mooney viscosity polybutadiene(JP-A 11-70187), solid golf balls molded from rubber compositionscomprising a polybutadiene having a Mooney viscosity of 50 to 69 andsynthesized using a nickel or cobalt catalyst in combination with apolybutadiene having a Mooney viscosity of 20 to 90 and synthesizedusing a lanthanide catalyst (JP-A 11-319148), solid golf balls moldedfrom compositions based on a rubber having a 1,2 vinyl content of atmost 2.0% and a weight-average molecular weight to number-averagemolecular weight ratio Mw/Mn of not more than 3.5 (JP-A 11-164912), golfballs molded from rubber compositions containing a high Mooney viscositypolybutadiene (JP-A 63-275356), and golf balls molded from rubbercompositions comprising polybutadiene having a high number-averagemolecular weight in admixture with polybutadiene having a lownumber-average molecular weight (JP-A 3-151985). However, none of theseprior-art golf balls truly have a good, soft feel upon impact and anexcellent spin performance that helps increase the distance traveled bythe ball.

Golf balls having a cover composed of a relatively hard inner layer anda relatively soft outer layer have already been disclosed in JP-A6-218078, JP-A 6-343718, JP-A 7-24085, JP-A 9-239068, JP-A 10-151226,JP-A 10-201880, JP-A 11-104273, JP-A 11-104271, and Japanese PatentApplications No. 2000-274807 and 2000-274843. However, furtherimprovements in distance are desired for the golf balls described in allof these specifications.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to providemulti-piece solid golf balls which are endowed with a good, soft feelwhen hit with a golf club and an excellent spin performance that helpsincrease the distance traveled by the ball when played.

The inventor has discovered that golf balls having a solid coreconsisting of a center core and an outer core, an inner cover layer andan outer cover layer, wherein the solid core is made of a rubbercomposition formulated from a particular type of base rubber combined inspecific proportions with certain other materials, and the center coreand the outer core and relationship therebetween are specified to thespecific hardness and the inner cover layer the outer cover layer andthe relationships therebetween are specified to the specific, exhibit agood synergy from optimization of the solid core materials and anappropriate distribution of hardness between the inner and outer coverlayers. Multi-piece solid golf balls thus constituted have a good, softfeel when hit with a golf club and an excellent spin performance thatenables the ball to travel further when played.

Accordingly, the invention provides a multi-piece solid golf ballcomprising a solid core consisting of a center core and an outer core,an inner cover layer and an outer cover layer. The solid core is moldedfrom a rubber composition comprising 100 parts by weight of a baserubber composed of (a) 20 to 100 wt % of a polybutadiene having acis-1,4 content of at least 60% and a 1,2 vinyl content of at most 2%,having a viscosity η at 25° C. as a 5 wt % solution in toluene of up to600 mPa·s, being synthesized using a rare-earth catalyst, in combinationwith (b) 0 to 80 wt % of a diene rubber other than component (a). Therubber composition includes also (c) 10 to 60 parts by weight of anunsaturated carboxylic acid or a metal salt thereof or both, (d) 0.1 to5 parts by weight of an organosulfur compound, (e) 5 to 80 parts byweight of an inorganic filler, and (f) 0.1 to 5 parts by weight of anorganic peroxide. The center core has a JIS-C hardness of 40 to 60 onits center and a JIS-C hardness of 55 to 75 on its surface and thedifference therebetween is at least 10, the outer core is harder thanthe surface hardness of the center core, the cross-sectional hardness of1 mm outside from the border between the center core and the outer coreis from 65 to 85 on a JIS-C hardness, the surface of the outer core hasa JIS-C hardness of 75 to 95.

In the multi-piece solid golf ball of the invention, it is generallyadvantageous for the inner cover layer to have a Shore D hardness of 50to 80 and the outer cover layer to have a Shore D hardness of 35 to 60,and the outer cover layer has a lower Shore D hardness than the innercover layer.

BRIEF DESCRIPTION OF THE FIGURE

The advantages, nature and various additional features of the inventionwill appear more fully upon consideration of the illustrative embodimentof the invention which is schematically set forth in the figures, inwhich:

FIG. 1 is a diagrammatical representation of a golf ball according tothe present invention.

DETAILED DESCRIPTION OF THE INVENTION

The golf ball of the invention includes a solid core made of a rubbercomposition in which the base rubber is at least partly polybutadiene.It is critical that the base rubber contain as component (a) a specificamount of a polybutadiene in which the cis-1,4 and 1,2 vinyl contents,the viscosity η at 25° C. as a 5 wt % solution in toluene, and therelationship between the Mooney viscosity and the polydispersity indexMw/Mn have each been optimized.

That is, the polybutadiene (a) has a cis-1,4 content of at least 60%,preferably at least 80%, more preferably at least 90%, and mostpreferably at least 95%; and has a 1,2 vinyl content of at most 2%,preferably at most 1.7%, more preferably at most 1.5%, and mostpreferably at most 1.3%. Outside of the above ranges, the resiliencedeclines.

The polybutadiene (a) must also have a viscosity η at 25° C. as a 5 wt %solution in toluene of not more than 600 mPa·s. “Viscosity η at 25° C.as a 5 wt % solution in toluene” refers herein to the value in mPa·sunits obtained by dissolving 2.28 g of the polybutadiene to be measuredin 50 ml of toluene and carrying out measurement with a specifiedviscometer at 25° C. using a standard solution for the viscometer (JISZ8809).

The polybutadiene (a) has a viscosity η at 25° C. as a 5 wt % solutionin toluene of not more than 600 mPa·s, preferably not more than 550mPa·s, more preferably not more than 500 mPa·s, even more preferably notmore than 450 mPa·s, and most preferably not more than 400 mPa·s. Toohigh a viscosity η lowers the workability of the rubber composition. Itis recommended that the viscosity η be at least 50 mPa·s, preferably atleast 100 mPa·s, more preferably at least 150 mPa·s, and most preferablyat least 200 mPa·s. Too low a viscosity η may lower the resilience.

In addition, it is recommended that the polybutadiene (a) satisfies therelationship:10B+5≦A≦10B+60,wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) of the polybutadieneand B is the ratio Mw/Mn between the weight-average molecular weight Mwand the number-average molecular weight Mn of the polybutadiene. A ispreferably at least 10B+7, more preferably at least 10B+8 and mostpreferably at least 10B+9, but preferably not more than 10B+) 55, morepreferably not more than 10B+50, and most preferably not more than10B+45. If A is too low, the resilience declines. On the other hand, ifA is too high, the workability of the rubber composition worsens.

It is recommended that the polybutadiene (a) have a Mooney viscosity(ML₁₊₄ (100° C.)) of at least 20, preferably at least 30, morepreferably at least 40, and most preferably at least 50, but not morethan 80, preferably not more than 70, more preferably not more than 65,and most preferably not more than 60.

The term “Mooney viscosity” used herein refers in each case to anindustrial index of viscosity as measured with a Mooney viscometer,which is a type of rotary plastometer (see JIS K6300). This value isrepresented by the symbol ML₁₊₄ (100° C.), wherein “M” stands for Mooneyviscosity, “L” stands for large rotor (L-type), “1+4” stands for apre-heating time of 1 minute and a rotor rotation time of 4 minutes, and“100° C.” indicates that measurement was carried out at a temperature of100° C.

The polybutadiene (a; must be synthesized using a rare-earth catalyst. Aknown rare-earth catalyst may be used for this purpose.

Examples of suitable catalysts include lanthanide series rare-earthcompounds, organoaluminum compounds, alumoxane, halogen-bearingcompounds, optionally in combination with Lewis bases.

Examples of suitable lanthanide series rare-earth compounds includehalides, carboxylates, alcoholates, thioalcoholates and amides of atomicnumber 57 to 71 metals.

Organoaluminum compounds that may be used include those of the formulaAlR¹R²R³ (wherein R¹, R² and R³ are each independently a hydrogen or ahydrocarbon residue of 1 to 8 carbons).

Preferred alumoxanes include compounds of the structures shown informulas (I) and (II) below. The alumoxane association complexesdescribed in Fine Chemical 23, No. 9, 5 (1994), J. Am. Chem. Soc. 115,4971 (1993), and J. Am. Chem. Soc. 117, 6465 (1995) are also acceptable.

In the above formulas, R⁴ is a hydrocarbon group having 1 to 20 carbonatoms, and n is 2 or a larger integer.

Examples of halogen-bearing compounds that may be used include aluminumhalides of the formula AlX_(n)R_(3-n) (wherein X is a halogen; R is ahydrocarbon residue of 1 to 20 carbons, such as an alkyl, aryl oraralkyl; and n is 1, 1.5, 2 or 3); strontium halides such as Me₃SrCl,Me₂SrCl₂, MeSrHCl₂ and MeSrCl₃ (wherein “Me” stands for methyl); andother metal halides such as silicon tetrachloride, tin tetrachloride andtitanium tetrachloride.

The Lewis base may be used to form a complex with the lanthanide seriesrare-earth compound. Illustrative examples include acetylacetone andketone alcohols.

In the practice of the invention, the use of a neodymium catalystcomposed in part of a neodymium compound as the lanthanide seriesrare-earth compound is advantageous because it enables a polybutadienerubber having a high cis-1,4 content and a low 1,2 vinyl content to beobtained at an excellent polymerization activity. Preferred examples ofsuch rare-earth catalysts include those mentioned in JP-A 11-35633.

For polymerization of butadiene in the presence of a rare-earth catalystin the form of a lanthanoid series rare-earth compound, in order thatthe cis content and the Mw/Mn fall in the above-mentioned ranges, themolar ratio of butadiene to lanthanoid series rare-earth compound ispreferably from 1,000/1 to 2,000,000/1, especially from 5,000/1 to1,000,000/1, and the molar ratio of AlR¹R²R³ to lanthanoid seriesrare-earth compound is preferably from 1/1 to 1,000/1, especially from3/1 to 500/1. Further, the molar ratio of halogen compound to lanthanoidseries rare-earth compound is preferably from 0.1/1 to 30/1, especiallyfrom 0.2/1 to 15/1. The molar ratio of Lewis base to lanthanoid seriesrare-earth compound is preferably from 0 to 30/1, especially from 1/1 to10/1. The polymerization of butadiene in the presence of a rare-earthcatalyst may be carried out either with or without the use of solvent,as by bulk polymerization or vapor phase polymerization. Thepolymerization temperature is generally in a range of −30° C. to 150°C., and preferably 10° C. to 100° C.

It is also possible for the polybutadiene (a) to be obtained bypolymerization using the above-described rare-earth catalyst, followedby the reaction of an end group modifier with active end groups on thepolymer.

Modified polybutadiene rubbers can be prepared by using end groupmodifiers (1) to (7) listed below, following the above polymerization.

-   (1) Compounds having an alkoxysilyl group to be reacted with the    polymer at active ends thereof. Suitable compounds having an    alkoxysilyl group are alkoxysilane compounds having at least one    epoxy or isocyanate group in a molecule, for example, epoxy    group-containing alkoxysilanes such as-   3-glycidyloxypropyltrimethoxysilane,-   3-glycidyloxypropyltriethoxysilane,-   (3-glycidyloxypropyl)methyldimethoxysilane,-   (3-glycidyloxypropyl)methyldiethoxysilane,-   β-(3,4-epoxycyclohexyl)trimethoxysilane,-   β-(3,4-epoxycyclohexyl)triethoxysilane,-   β-(3,4-epoxycyclohexyl)methyldimethoxysilane,-   β-(3,4-epoxycyclohexyl)ethyldimethoxysilane, condensates of    3-glycidyloxypropyltrimethoxysilane, and condensates of    (3-glycidyloxypropyl)methyldimethoxysilane; and isocyanato    group-containing alkoxysilanes such as-   3-isocyanatopropyltrimethoxysilane,-   3-isocyanatopropyltriethoxysilane,-   (3-isocyanatopropyl)methyldimethoxysilane,-   (3-isocyanatopropyl)methyldiethoxysilane, condensates of    3-isocyanatopropyltrimethoxysilane, and condensates of    (3-isocyanatopropyl)methyldimethoxysilane.

When a compound having an alkoxysilyl group is reacted to active ends ofthe polymer, a Lewis acid may be added for promoting the reaction. TheLewis acid added serves as a catalyst to promote coupling reaction forimproving the cold flow and storage stability of the modified polymer.Examples of the Lewis acid include dialkyltin dialkylmaleates,dialkyltin dicarboxylates, and aluminum trialkoxides.

-   (2) Halogenated organometallic compounds, halogenated metallic    compounds and organometallic compounds of the general formulas: R⁵    _(n)M′X_(4-n), M′X₄, M′X₃, R⁵ _(n)M′ (—R⁶—COOR⁷)_(4-n) or R⁵ _(n)M′    (—R⁶—COR⁷)_(4-n) (wherein R⁵ and R⁶ are each independently a    hydrocarbon group of 1 to 20 carbon atoms; R⁷ is a hydrocarbon group    of 1 to 20 carbon atoms which may contain a carbonyl or ester moiety    on a side chain; M′ is a tin atom, silicon atom, germanium atom or    phosphorus atom; X is a halogen atom; and n is an integer from 0 to    3).-   (3) Heterocumulene compounds containing on the molecule a Y═C=Z    linkage (wherein Y is a carbon atom, oxygen atom, nitrogen atom or    sulfur atom; and Z is an oxygen atom, nitrogen atom or sulfur atom).-   (4) Three-membered heterocyclic compounds containing on the molecule    the following linkage:

(wherein Y is an oxygen atom, nitrogen atom or sulfur atom).

-   (5) Halogenated isocyano compounds.-   (6) Carboxylic acids, acid halides, ester compounds, carbonate    compounds or acid anhydrides of the formulas: R⁸—(COOH)_(m),    R⁹(COX)_(m), R¹⁰—(COO—R¹¹)_(m), R¹²—OCOO—R¹³, R¹⁴—(COOCO—R¹⁵)_(m) or    the following formula:

(wherein R⁸ to R¹⁶ are each independently a hydrocarbon group of 1 to 50carbon atoms; X is a halogen atom; and m is an integer from 1 to 5); and

-   (7) Carboxylic acid metal salts of the formula: R¹⁷ _(l)M″    (OCOR¹⁸)_(4-l), R¹⁹ _(l)M″ (OCO—R²⁰—COOR²¹)_(4-l) or the following    formula:

(wherein R¹⁷ to R²³ are each independently a hydrocarbon group of 1 to20 carbon atoms, M″ is a tin atom, silicon atom or germanium atom; and 1is an integer from 0 to 3).

Illustrative examples of the end group modifiers of types (1) to (7)above and methods for their reaction are described in, for instance,JP-A 11-35633, JP-A 7-268132 and JP-A 2002-293996.

In the practice of the invention, component (a) is included in the baserubber in an amount of at least 20 wt %, preferably at least 25 wt %,more preferably at least 30 wt %, and most preferably at least 35 wt %.The upper limit is 100 wt %, preferably not more than 90 wt %, morepreferably not more than 80 wt %, and most preferably not more than 70wt %.

In addition to component (a), the base rubber may include also a dienerubber (b) insofar as the objects of the invention are attainable.Specific examples of the diene rubbers (b) include polybutadiene rubber,styrene-butadiene rubber (SBR), natural rubber, polyisoprene rubber, andethylene-propylene-diene rubber (EPDM). Any one or combination of two ormore thereof may be used.

The diene rubber (b) is included together with component (a) in the baserubber in an amount of at least 0 wt %, preferably at least 10 wt %,more preferably at least 20 wt %, and most preferably at least 30 wt %,but not more than 80 wt %, preferably not more than 75 wt %, morepreferably not more than 70 wt %, and most preferably not more than 65wt %.

In the practice of the invention, it is preferable for component (b) toinclude a polybutadiene rubber, and especially one for which the cis-1,4and 1,2 vinyl contents, the Mooney viscosity, and the relationshipbetween the Mooney viscosity and η have each been optimized. Thepolybutadiene serving as component (b) is referred to as “secondpolybutadiene” in order to distinguish it from the polybutadiene servingas component (a).

It is recommended that the second polybutadiene in component (b) have acis-1,4 content of at least 60%, preferably at least 80%, morepreferably at least 90%, and most preferably at least 95%, and that ithave a 1,2 vinyl content of at most 5%, preferably at most 4.5%, morepreferably at most 4.0%, and most preferably at most 3.5%.

It is recommended that the second polybutadiene have a Mooney viscosityof at least 10, preferably at least 20, more preferably at least 25, andmost preferably at least 30, but not more than 55, preferably not morethan 50, and most preferably not more than 45.

In the practice of the invention, it is recommended that the secondpolybutadiene be one that has been synthesized using a Group VIIIcatalyst. Exemplary Group VIII catalysts include nickel catalysts andcobalt catalysts.

Examples of suitable nickel catalysts include single-component systemssuch as nickel-kieselguhr, binary systems such as Raney nickel/titaniumtetrachloride, and ternary systems such as nickelcompound/organometallic compound/boron trifluoride etherate. Exemplarynickel compounds include reduced nickel on a carrier, Raney nickel,nickel oxide, nickel carboxylate and organonickel complexes. Exemplaryorganometallic compounds include trialkylaluminum compounds such astriethylaluminum, tri-n-propylaluminum, triisobutylaluminum andtri-n-hexylaluminum; alkyllithium compounds such as n-butyllithium,sec-butyllithium, tert-butyllithium and 1,4-dilithiumbutane; anddialkylzinc compounds such as diethylzinc and dibutylzinc.

Examples of suitable cobalt catalysts include the following composed ofcobalt or cobalt compounds: Raney cobalt, cobalt chloride, cobaltbromide, cobalt iodide, cobalt oxide, cobalt sulfate, cobalt carbonate,cobalt phosphate, cobalt phthalate, cobalt carbonyl, cobaltacetylacetonate, cobalt diethyldithiocarbamate, cobalt anilinium nitriteand cobalt dinitrosyl chloride. It is particularly advantageous to usethe above in combination with a dialkylaluminum monochloride such asdiethylaluminum monochloride or diisobutylaluminum monochloride; atrialkylaluminum such as triethylaluminum, tri-n-propylaluminum,triisobutylaluminum or tri-n-hexylaluminum; an alkyl aluminumsesquichloride such as ethylaluminum sesquichloride; or aluminumchloride.

Polymerization using the Group VIII catalysts described above, andespecially a nickel or cobalt catalyst, can generally be carried out bya process in which the catalyst is continuously charged into the reactortogether with the solvent and butadiene monomer, and the reactionconditions are suitably selected from a temperature range of 5 to 60° C.and a pressure range of atmospheric pressure to 70 plus atmospheres, soas to yield a product having the above-indicated Mooney viscosity.

It is also desirable for the second polybutadiene in component (b) tosatisfy the relationship:20A−750≦η≦20A−550,wherein η is the viscosity of the second polybutadiene at 25° C. as a 5wt % solution in toluene and A is the Mooney viscosity (ML₁₊₄ (100° C.))of the second polybutadiene. The viscosity η is preferably at least20A−700, more preferably at least 20A−680, and most preferably at least20A−650, but preferably not more than 20A−560, more preferably not morethan 20A−580, and most preferably not more than 20A−590. The use of apolybutadiene having such an optimized relationship of η and A, thatsuggests the high linearity of polybutadiene molecules, is effective forconferring better resilience and workability.

The second polybutadiene generally accounts for at least 30 wt %,preferably at least 50 wt %, and most preferably at least 70 wt %, andup to 100 wt %, preferably up to 90 wt %, and most preferably up to 80wt %, of the diene rubber (b). By including the second polybutadienewithin component (b) in the foregoing range, even better extrudabilityand hence, workability during manufacture can be conferred.

The solid core in the golf balls of the invention is molded from arubber composition containing as essential components specific amountsof (c) an unsaturated carboxylic acid and/or metal salt thereof, (d) anorganosulfur compound, (e) an inorganic filler and (f) an organicperoxide per 100 parts by weight of the base rubber.

Specific examples of unsaturated carboxylic acids that may be used ascomponent (c) include acrylic acid, methacrylic acid, maleic acid andfumaric acid. Acrylic acid and methacrylic acid are especiallypreferred.

Specific examples of unsaturated carboxylic acid metal salts that may beused as component (c) include the zinc and magnesium salts ofunsaturated fatty acids such as zinc methacrylate and zinc acrylate.Zinc acrylate is especially preferred.

The unsaturated carboxylic acid and/or metal salt thereof used ascomponent (c) is included in an amount, per 100 parts by weight of thebase rubber, of at least 10 parts by weight, preferably at least 15parts by weight, and most preferably at least 20 parts by weight, butnot more than 60 parts by weight, preferably not more than 50 parts byweight, more preferably not more than 45 parts by weight, and mostpreferably not more than 40 parts by weight. Too much component (c)results in excessive hardness, giving the golf ball a feel upon impactthat is difficult for the player to endure. On the other hand, toolittle component (c) undesirably lowers the resilience.

The organosulfur compound (d) of the rubber composition is essential forimparting good resilience. Exemplary organosulfur compounds includethiophenol, thionaphthol, halogenated thiophenols, and metal saltsthereof. Specific examples include pentachlorothiophenol,pentafluorothiophenol, pentabromothiophenol, p-chlorothiophenol, andzinc salts thereof, such as the zinc salt of pentachlorothiophenol; andorganosulfur compounds having 2 to 4 sulfurs, such asdiphenylpolysulfides, dibenzylpolysulfides, dibenzoylpolysulfides,dibenzothiazoylpolysulfides and dithiobenzoylpolysulfides.Diphenyldisulfide and the zinc salt of pentachlorothiophenol areespecially preferred.

The organosulfur compound (d) is included in an amount, per 100 parts byweight of the base rubber, of at least 0.1 part by weight, preferably atleast 0.2 part by weight, and most preferably at least 0.5 part byweight, but not more than 5 parts by weight, preferably not more than 4parts by weight, more preferably not more than 3 parts by weight, andmost preferably not more than 2 parts by weight. Too much organosulfurcompound results in an excessively low hardness, whereas too littlemakes it impossible to enhance the resilience.

Examples of inorganic fillers that may be used as component (e) includezinc oxide, barium sulfate and calcium carbonate. The inorganic filler(e) is included in an amount, per 100 parts by weight of the baserubber, of at least 5 parts by weight, preferably at least 7 parts byweight, more preferably at least 10 parts by weight, and most preferablyat least 13 parts by weight, but not more than 80 parts by weight,preferably not more than 50 parts by weight, more preferably not morethan 45 parts by weight, and most preferably not more than 40 parts byweight. Too much or too little inorganic filler makes it impossible toachieve a golf ball core having an appropriate weight and good reboundcharacteristics.

The organic peroxide (f) may be a commercial product, suitable examplesof which include Percumil D (manufactured by NOF Corporation), Perhexa3M (manufactured by NOF Corporation) and Luperco 231XL (manufactured byAtochem Co.). If necessary, two or more different organic peroxides maybe mixed and used together.

The organic peroxide (f) is included in an amount, per 100 parts byweight of the base rubber, of at least 0.1 part by weight, preferably atleast 0.3 part by weight, more preferably at least 0.5 part by weight,and most preferably at least 0.7 part by weight, but not more than 5parts by weight, preferably not more than 4 parts by weight, morepreferably not more than 3 parts by weight, and most preferably not morethan 2 parts by weight. Too much or too little organic peroxide makes itimpossible to achieve a ball having a good feel upon impact and gooddurability and rebound characteristics.

If necessary, the rubber composition may also include an antioxidant,suitable examples of which include such commercial products as NocracNS-6, Nocrac NS-30 (both made by Ouchi Shinko Chemical Industry Co.,Ltd.), and Yoshinox 425 (made by Yoshitomi Pharmaceutical Industries,Ltd.). The use of such an antioxidant in an amount, per 100 parts byweight of the base rubber, of at least 0 part by weight, preferably atleast 0.05 part by weight, more preferably at least 0.1 part by weight,and most preferably at least 0.2 part by weight, but not more than 3parts by weight, preferably not more than 2 parts by weight, morepreferably not more than 1 part by weight, and most preferably not morethan 0.5 part by weight, is desirable for achieving good reboundcharacteristics and durability.

As shown in FIG. 1, the solid core consists of a center core 1 and anouter core 2 around the center core 1. That construction of the solidcore realizes the reduction of the spin rate when hitting, therebyincreasing the flight distance of the golf balls substantially.

It is recommended that the center core is formed to a diameter of atleast 15 mm, preferably at least 20 mm, more preferably at least 22 mm,most preferably at least 24, but not more than 36 mm, preferably notmore than 33 mm, more preferably not more than 30 mm, most preferablynot more than 28 mm.

It is also recommended that the center hardness of the center core on aJIS-C scale is at least 40, preferably at least 42, more preferably atleast 44, most preferably at least 46, but not more than 60, preferablynot more than 58, more preferably not more than 56, most preferably notmore than 54. It is further recommended that the surface hardness of thecenter core on a JIS-C scale is at least 55, preferably at least 57,more preferably at least 59, most preferably at least 61, but not morethan 75, preferably not more than 73, more preferably not more than 71,most preferably not more than 69.

In the center core, the difference between the center hardness and thesurface on a JIS-C scale is at least 10. It is recommended that thedifference of the hardness on a JIS-C scale therebetween is at least 12,preferably at least 13, more preferably at least 15, but not more than25, preferably not more than 23, more preferably not more than 20.

It is recommended that the outer core has a thickness of at least 1.5mm, preferably at least 2 mm, more preferably at least 2.5 mm, mostpreferably 3 mm, but not more than 10 mm, preferably not more than 9 mm,more preferably not more than 8 mm, most preferably not more than 7 mm.

The outer core is harder than the surface hardness of the center core.In particular, it is recommended that the difference between thehardness of the outer core and the surface hardness of the center coreis at least 2, preferably at least 3, more preferably at least 4, butnot more than 30, preferably not more than 20, more preferably not morethan 15. It is recommended that a surface hardness of the outer core ona JIS-C scale is at least 75, preferably at least 77, more preferably atleast 79, most preferably at least 81, but not more than 95, preferablynot more than 93, more preferably not more than 91, most preferably notmore than 89.

The cross-sectional hardness of 1 mm outside from the border between thecenter core and the outer core on a JIS-C scale is at least 65,preferably at least 68, more preferably at least 71, most preferably atleast 74, but not more than 85, preferably not more than 83, morepreferably not more than 80, most preferably not more than 77.

In the present invention, the center core and the outer core are formedby an injection molding process and a compression molding process,respectively. It is preferred that the non-vulcanized rubber compositionfor an outer core is filled into the cavity of the mold used for apreparation of hemispherical cups and is subjected to semi-vulcanizationat 100 to 160° C. for 1 to 10 minutes to form a pair of hemisphericalcups in the state of semi-vulcanization. Then the pair of cups arefitted each other and the pair of cups cover the center core to preparea solid core consisting of the center core and the outer core by a pressmolding process into a cavity of the mold at 100 to 200° C. for 5 to 20minutes.

The golf ball of the invention is a multi-piece solid golf ball having acover composed of at least two layers which are referred to herein asthe “inner cover layer” 3 and the “outer cover layer” 4. Such coverlayers can be produced from known cover stock. The cover stocks used tomake both cover layers in the inventive golf ball may be composedprimarily of a thermoplastic or thermoset polyurethane elastomer,polyester elastomer, ionomer resin, ionomer resin having a relativelyhigh degree of neutralization, polyolefin elastomer or mixture thereof.Any one or mixture of two or more thereof may be used, although the useof a thermoplastic polyurethane elastomer, ionomer resin or ionomerresin having a relatively high degree of neutralization is especiallypreferred.

Illustrative examples of thermoplastic polyurethane elastomers that maybe used for the above purpose include commercial products in which thediisocyanate is an aliphatic or aromatic compound, such as Pandex T7298,Pandex T7295, Pandex T7890, Pandex TR3080, Pandex T8290, Pandex T8295and Pandex T1188 (all manufactured by DIC Bayer Polymer, Ltd.).Illustrative examples of suitable commercial ionomer resins includeSurlyn 6320, Surlyn 8945, Surlyn 9945 and Surlyn 8120 (both products ofE.I. du Pont de Nemours and Co., Inc.), and Himilan 1706, Himilan 1605,Himilan 1855, Himilan 1557, Himilan 1601 and Himilan AM7316 (allproducts of DuPont-Mitsui Polychemicals Co., Ltd.).

Together with the primary material described above, the cover stock mayinclude also, as an optional material, polymers (e.g., thermoplasticelastomers) other than the foregoing. Specific examples of polymers thatmay be included as optional constituents include polyamide elastomers,styrene block elastomers, hydrogenated polybutadienes and ethylene-vinylacetate (EVA) copolymers.

The multi-piece solid golf ball of the invention can be manufactured byany suitable known method without particular limitation. In onepreferred method, the solid core is placed within a given injectionmold, following which a predetermined method is used to successivelyinject over the core the above-described inner and outer cover layermaterials. In another preferred method, each of the cover stocks isformed into a pair of half cups, and the resulting pairs aresuccessively placed over the solid core and compression molded.

In the golf balls of the invention, it is critical that the inner coverlayer has a Shore D hardness of 50 to 80 and the outer cover layer has aShore D hardness of 35 to 60, and the outer cover layer has a lowerShore D hardness than the inner cover layer.

It is recommended that the inner cover layer have a Shore D hardness ofat least 51, preferably at least 52, and most preferably at least 53,but not more than 75, preferably not more than 70, and most preferablynot more than 65.

It is recommended that the outer cover layer have a Shore D hardness ofat least 40, preferably at least 45, and most preferably at least 48,but not more than 58, preferably not more than 56, and most preferablynot more than 54.

As noted above, in the practice of the invention the outer cover layermust have a lower Shore D hardness than the inner cover layer. It isadvantageous for the inner and outer cover layers to have a differencein Shore D hardness of at least 2, preferably at least 5, morepreferably at least 7, and most preferably at least 9 Shore D hardnessunits, but not more than 30, preferably not more than 25, and mostpreferably not more than 20 Shore D hardness units.

It is recommended that the inner and outer cover layers have arespective thickness of at least 0.2 mm, preferably at least 0.5 mm,more preferably at least 0.8 mm, most preferably at least 1.0 mm. It isrecommended that the inner cover layer has a thickness of not more than3.0 mm, preferably not more than 2.5 mm, more preferably not more than2.0 mm, most preferably not more than 1.5 mm. It is also recommendedthat the outer cover layer has a thickness of not more than 2.0 mm,preferably not more than 1.8 mm, more preferably not more than 1.5 mm,most preferably not more than 1.2 mm.

The multi-piece solid golf ball of the invention can be manufactured forcompetitive use by imparting the ball with a diameter and weight whichconform with the Rules of Golf; that is, a diameter of at least 42.67 mmand a weight of not more than 45.93 g. It is recommended that thediameter be no more than 44.0 mm, preferably no more than 43.5 mm, andmost preferably no more than 43.0 mm; and that the weight be at least44.5 g, preferably at least 45.0 g, more preferably at least 45.1 g, andmost preferably at least 45.2 g.

Multi-piece solid golf balls according to the present invention have agood, soft feel upon impact and an excellent spin performance thatenable the ball to travel a greater distance when played.

EXAMPLES

The following examples and comparative examples are provided toillustrate the invention, and are not intended to limit the scopethereof.

Examples 1–4 & Comparative Examples 1–6

The center core materials and the outer core materials shown in Table 2were formulated in the indicated amounts per 100 parts by weight ofpolybutadiene material composed of polybutadiene types (1) to (5) belowin the proportions shown in Table 1. The resulting center coreformulations were blended in a kneader or on a roll mill and then werecompression molded at 160° C. for 15 minutes to form the center core.The resulting outer core formulations were blended and then werecompression molded at 130° C. for 3 minutes to form the outer core inform of the pair of cups. The resulting pairs are successively placedover the center core and compression molded one again at 170° C. for 10minutes to obtain the solid core having two layers consisting of thecenter core and the outer core. Types of Polybutadiene:

-   (1) BR01, made by JSR Corporation-   (2) BR11, made by JSR Corporation-   (3) HCBN-4, an experimental grade of polybutadiene made by JSR    Corporation-   (4) Experimental grade #9100081 made by Firestone-   (5) BR 730, made by JSR Corporation

TABLE 1 cis-1,4 1,2 vinyl Mooney content, content, viscosity Mw/Mn TypeCatalyst % % (A) (B) η 10B + 5 10B + 60 20A − 550 Polybutadiene (1) Ni96 2.5 44 4.2 150 47 102 330 (2) Ni 96 2 44 4.4 270 49 104 330 (3) Nd 961.1 44 3.5 390 40 95 330 (4) Nd 95 1.5 56 2.6 370 31 86 570 (5) Nd 961.2 55 3 400 35 90 550

TABLE 2 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 Center CoreCenter Polybutadiene (1) 100 Core Polybutadiene (2) 30 formulationPolybutadiene (3) 100 100 100 100 (pbw) Polybutadiene (4) 70Polybutadiene (5) 100 100 100 100 Dicumyl peroxide 0.3 1.4 0.3 0.7 0.70.7 0.3 0.3 0.3 0.3 1,1-bis(t-butylperoxy)- 0.12 0.12 0.3 0.3 0.12 0.120.12 0.12 3,3,5-trimethylcyclohexane Zinc oxide 17.5 16.5 17.5 11.5 1814.5 22 15.5 28 28 Antioxidant 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1Zinc acrylate 26 25 26 31 26 34 16 33 25 25 Zinc salt of 1 1 1 2 1 1 1 01 1 pentachlorothiophenol Outer Core Outer Polybutadiene (1) 100 noneCore Polybutadiene (2) 30 formulation Polybutadiene (3) 100 100 100 100(pbw) Polybutadiene (4) 70 Polybutadiene (5) 100 100 100 Dicumylperoxide 0.3 1.4 0.3 0.7 0.7 0.3 0.3 0.3 0.3 1,1-bis(t-butylperoxy)-0.12 0.12 0.3 0.3 0.12 0.12 0.12 0.12 3,3,5-trimethylcyclohexane Zincoxide 14.5 12 14.5 7.5 14.5 14.5 12 24.5 24.5 Antioxidant 0.1 0.1 0.10.1 0.1 0.1 0.1 0.1 Zinc acrylate 34 36 34 40 34 34 41 34 34 Zinc saltof 1 1 1 2 1 1 0 1 1 pentachlorothiophenolRebound:

The initial velocity of the solid cores consisting the center cores andthe outer cores was measured with the same type of initial velocityinstrument as used by the official regulating body—the United StatesGolf Association (USGA). Each rebound value shown in Table 4 is thedifference between the initial velocity of the solid core obtained inthat particular example and the initial velocity of the solid coreobtained in Comparative Example 4.

In each example, the resulting solid core was placed in a given mold andthe appropriate resin shown in Table 3 was injection-molded over thecore, thereby producing an inner layer-covered core. The covered corewas then transferred to a given mold, and the appropriate resin shown inTable 3 was injection molded over the covered core, yielding a solidgolf ball having a diameter of about 42.7 mm and a weight of about 45.3g. Trade names appearing in Table 3 are described below.

-   Himilan: An ionomer resin produced by DuPont-Mitsui Polychemicals    Co., Ltd.-   Surlyn: An ionomer resin produced by E.I. du Pont de Nemours and Co.-   Dynaron: An E-EB-E block copolymer produced by JSR Corporation-   Pandex: A polyurethane elastomer produced by Bayer-DIC Polymer, Ltd.

The properties of the resulting golf balls were determined as describedbelow. The results are shown in Table 4.

Material Properties:

The Shore D hardnesses of the inner cover layer and the outer coverlayer were measured with a durometer by the test method described inASTM D2240.

Golf Ball Properties:

The carry and total distance were measured when the ball was hit at ahead speed (HS) of 50 m/s with a driver (No. 1 Wood) mounted on a swingmachine.

Feel:

The feel of the ball when actually shot with a driver (No. 1 Wood) andputter was rated by five professional and five top-caliber amateurgolfers as “Too hard,” “Slightly hard,” “Good” or “Slightly soft,” “Toosoft.” The rating assigned most often to a particular ball was used asthat ball's overall rating.

TABLE 3 A B C D E F Formu- Himilan 1706 50 lation Himilan 1605 50 (pbw)Himilan 1557 20 Himilan 1855 30 Surlyn 8945 35 Surlyn 9945 35 Surlyn8120 100 50 Dynaron 6100P 30 Pandex T8290 50 Pandex T8295 50 100Titanium 4 4 4 2.7 2.7 4 dioxide

TABLE 4 Example Comparative Example 1 2 3 4 1 2 3 4 5 6 Sold CenterOuter diameter (mm) 28.9 26.0 28.9 25.2 28.9 36.3 28.9 28.9 28.9 28.9Core Core Weight (g) 14.5 10.5 14.5 9.5 14.5 28.8 14.5 14.5 15.3 15.3Specific gravity 1.15 1.14 1.15 1.13 1.15 1.15 1.15 1.15 1.21 1.21Center hardness 50.0 50.0 50.0 59.0 50.0 63.0 35.0 63.0 50.0 50.0 JIS-Cscale Surface Hardness 67.0 65.0 67.0 74.0 67.0 82.0 50.0 82.0 67.0 67.0JIS-C scale Cross-Sectional 17.0 15.0 17.0 15.0 17.0 19.0 15.0 19.0 17.017.0 hardness difference JIS-C scale Outer Thickness (mm) 3.7 5.2 3.77.0 3.7 3.7 3.7 3.7 3.7 core Portion of 1 mm 72.0 75.0 72.0 78.0 72.072.0 85.0 72.0 72.0 outside from the border the center core and theouter core Surface hardness 82.0 85.0 82.0 91.0 82.0 82.0 95.0 82.0 82.0JIS-C scale The hardness difference 5.0 10.0 5.0 4.0 5.0 22.0 3.0 5.05.0 between the outer core and the center core surface on JIS C scaleSpecific gravity 1.15 1.14 1.15 1.13 1.15 1.15 1.15 1.21 1.21 Rebound(m/s) +0.6 +0.4 +0.5 +0.3 0 −0.2 −0.8 0 +0.5 +0.5 Inner Type A A B A A AA A C C cover Shore D hardness 63 63 56 63 63 63 63 63 45 45 layerSpecific gravity 0.98 0.98 0.97 0.98 0.98 0.98 0.98 0.98 0.98 0.98Thickness (mm) 1.7 1.2 1.7 1.0 1.7 1.7 1.7 1.7 1.7 1.7 Outer Type D D ED D D D D F A cover Shore D hardness 47 47 51 47 47 47 47 47 53 63 layerSpecific gravity 1.18 1.18 1.18 1.18 1.18 1.18 1.18 1.18 0.98 0.98Thickness (mm) 1.5 1.1 1.5 0.8 1.5 1.5 1.5 1.5 1.5 1.5 Golf When hitCarry (m) 229.1 226.8 227.9 226.1 223.0 221.2 215.2 223.2 217.9 222.3ball with Total 260.6 258.4 259.8 257.5 254.1 251.7 248.5 252.9 248.6253.5 prop- No. 1 distance (m) erties Wood Spin rate 3090 3081 3077 30533093 3211 2865 3368 3283 3151 at HS of (rpm) 50 m/s Feel on good goodgood Slightly good Slightly too too too good impact hard hard soft hardsoft Spin rate on approach shot 6323 6298 6259 6281 6318 6320 6262 63556218 4318 (sand wedge; HS 20 m/s) Feel of ball good good good good goodgood good Slightly too too when hit with putter hard soft hard

Japanese Patent Application No. 2001-163238 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A multi-piece solid golf ball comprising a solid core consisting of acenter core and an outer core, an inner cover layer and an outer coverlayer, wherein the solid core is molded from a rubber compositioncomprising 100 parts by weight of a base rubber composed of (a) 20 to100 wt % of a polybutadiene having a cis-1,4 content of at least 60% anda 1,2 vinyl content of at most 2%, having a viscosity η at 25° C. as a 5wt % solution in toluene of up to 600 mPa·s, being synthesized using arare-earth catalyst, in combination with (b) 0 to 80 wt % of a dienerubber other than component (a), (c) 10 to 60 parts by weight of anunsaturated carboxylic acid or a metal salt thereof or both, (d) 0.1 to5 parts by weight of an organosulfur compound, selected from a groupconsisting of thiophenol, thionaphthol, halogenated thiophenols, andmetal salts thereof, (e) 5 to 80 parts by weight of an inorganic filler,and (f) 0.1 to 5 parts by weight of an organic peroxide; the center corehas a JIS-C hardness of 40 to 60 on its center and a JIS-C hardness of55 to 75 on its surface and the difference therebetween is at least 10,the outer core is harder than the surface hardness of the center core,the cross-sectional hardness of 1 mm outside from the border between thecenter core and the outer core is from 65 to 85 on a JIS-C hardness, asurface of the outer core has a JIS-C hardness of 75 to 95, the innercover layer has a Shore D hardness of 50 to 80, the outer cover layerhas a Shore D hardness of 35 to 60, and the outer cover layer has alower Shore D hardness than the inner cover layer.
 2. The golf ball ofclaim 1, wherein the polybutadiene (a) satisfies relationship:10B+5≦A≦10B+60, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) ofthe polybutadiene and B is the ratio Mw/Mn between the weight-averagemolecular weight Mw and the number-average molecular weight Mn of thepolybutadiene.
 3. The golf ball of claim 1, wherein the diene rubber (b)includes 30 to 100 wt % of a second polybutadiene which has a cis-1,4content of at least 60% and a 1,2 vinyl content of at most 5%, has aMooney viscosity (ML₁₊₄ (100° C.)) of not more than 55, and satisfiesthe relationship:η≦20A−550, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) of thesecond polybutadiene and η is the viscosity of the second polybutadiene,in mPa·s, at 25° C. as a 5 wt % solution in toluene.
 4. The golf ball ofclaim 3, wherein the second polybutadiene in component (b) issynthesized using a Group VIII catalyst.
 5. The golf ball of claim 1,wherein the center core has a diameter of 15 to 36 mm and the outer corehas a thickness of 1.5 to 10 mm, and the inner cover layer has athickness of 0.2 to 3.0 mm and the outer cover layer has a thickness of0.2 to 2.0 mm.
 6. The golf ball of claim 1, wherein the organosulfurcompound is the zinc salt of pentachlorothiophenol.
 7. The golf ball ofclaim 1, wherein said core has a deflection under a load of 100 kg inthe range of 2 to 6 mm.
 8. The golf ball of claim 1, wherein said corehas a deflection under a load of 100 kg in the range of 2.5 to 5.5 mm.9. The golf ball of claim 1, wherein said core has a deflection under aload of 100 kg in the range of 2.8 to 5 mm.
 10. The golf ball of claim1, wherein said core has a deflection under a load of 100 kg in therange of 3.2 to 4.5 mm.
 11. The golf ball of claim 1, wherein thediameter of the core is in the range of 30 to 40 mm.
 12. The golf ballof claim 1, wherein the core has a specific gravity in the range of 0.9to 1.4.
 13. The golf ball of claim 1, wherein the inner cover layer hasa Shore D hardness in the range of 51 to
 75. 14. The golf ball of claim1, wherein the inner cover layer has a Shore D hardness in the range of52 to
 70. 15. The golf ball of claim 1, wherein the inner cover layerhas a Shore D hardness in the range of 53 to
 65. 16. The golf ball ofclaim 1, wherein the outer cover layer has a Shore D hardness in therange of 40 to
 58. 17. The golf ball of claim 1, wherein the outer coverlayer has a Shore D hardness in the range of 45 to
 56. 18. The golf ballof claim 1, wherein the outer cover layer has a Shore D hardness in therange of 48 to
 54. 19. The golf ball of claim 1, wherein the differencein Shore D hardness between said inner cover layer and said outer coverlayer is at least
 2. 20. The golf ball of claim 1, wherein a centerhardness of said center core is in the range of 42 to 58 on the JIS-Chardness scale.
 21. The golf ball of claim 1, wherein a center hardnessof said center core is in the range of 44 to 56 on the JIS-C hardnessscale.
 22. The golf ball of claim 1, wherein a center hardness of saidcenter core is in the range of 46 to 54 on the JIS-C hardness scale. 23.The golf ball of claim 1, wherein a surface hardness of said center coreis in the range of 55 to 69 on the JIS-C hardness scale.
 24. The golfball of claim 1, wherein a surface hardness of said center core is inthe range of 57 to 73 on the JIS-C hardness scale.
 25. The golf ball ofclaim 1, wherein a surface hardness of said center core is in the rangeof 59 to 71 on the JIS-C hardness scale.
 26. The golf ball of claim 1,wherein a surface hardness of said center core is in the range of 61 to69 on the JIS-C hardness scale.
 27. The golf ball of claim 1, wherein adifference in JIS-C hardness between a center hardness of said centercore and a surface hardness of said center core is at least
 15. 28. Thegolf ball of claim 1, wherein a difference in JIS-C hardness between acenter hardness of said center core and a surface hardness of saidcenter core is in the range of 12 to
 25. 29. The golf ball of claim 1,wherein a difference in JIS-C hardness between a center hardness of saidcenter core and a surface hardness of said center core is in the rangeof 13 to
 23. 30. The golf ball of claim 1, wherein a difference in JIS-Chardness between a center hardness of said center core and a surfacehardness of said center core is in the range of 15 to
 20. 31. Amulti-piece solid golf ball comprising a solid core consisting of acenter core and an outer core, an inner cover layer and an outer coverlayer, wherein the solid core is molded from a rubber compositioncomprising 100 parts by weight of a base rubber composed of (a) 20 to100 wt % of a polybutadiene having a cis-1,4 content of at least 60% anda 1,2 vinyl content of at most 2%, having a viscosity η at 25° C. as a 5wt % solution in toluene of up to 600 mPa·s, being synthesized using arare-earth catalyst, in combination with (b) 0 to 80 wt % of a dienerubber other than component (a), (c) 10 to 60 parts by weight of anunsaturated carboxylic acid or a metal salt thereof or both, (d) 0.1 to5 parts by weight of an organosulfur compound, selected from a groupconsisting of pentachlorothiophenol, pentafluorothiophenol,pentabromothiophenol, p-chlorothiophenol, and zinc salts thereof, (e) 5to 80 parts by weight of an inorganic filler, and (f) 0.1 to 5 parts byweight of an organic peroxide; the center core has a JIS-C hardness of40 to 60 on its center and a JIS-C hardness of 55 to 75 on its surfaceand the difference therebetween is at least 10, the outer core is harderthan the surface hardness of the center core, the cross-sectionalhardness of 1 mm outside from the border between the center core and theouter core is from 65 to 85 on a JIS-C hardness, a surface of the outercore has a JIS-C hardness of 75 to 95, the inner cover layer has a ShoreD hardness of 50 to 80, the outer cover layer has a Shore D hardness of35 to 60, and the outer cover layer has a lower Shore D hardness thanthe inner cover layer.
 32. The golf ball of claim 31 wherein thepolybutadiene (a) satisfies relationship: 10B+5≦A≦10B+60, wherein A isthe Mooney viscosity (ML₁₊₄ (100° C.)) of the polybutadiene and B is theratio Mw/Mn between the weight-average molecular weight Mw and thenumber-average molecular weight Mn of the polybutadiene.
 33. The golfball of claim 31, wherein the diene rubber (b) includes 30 to 100 wt %of a second polybutadiene which has a cis-1,4 content of at least 60%and a 1,2 vinyl content of at most 5%, has a Mooney viscosity (ML₁₊₄(100° C.)) of not more than 55, and satisfies the relationship:η≦20A−550, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) of thesecond polybutadiene and η is the viscosity of the second polybutadiene,in mPa·s, at 25° C. as a 5 wt % solution in toluene.
 34. The golf ballof claim 33, wherein the second polybutadiene in component (b) issynthesized using a Group VIII catalyst.
 35. The golf ball of claim 31,wherein the center core has a diameter of 15 to 36 mm and the outer corehas a thickness of 1.5 to 10 mm, and the inner cover layer has athickness of 0.2 to 3.0 mm and the outer cover layer has a thickness of0.2 to 2.0 mm.
 36. A multi-piece solid golf ball comprising a solid coreconsisting of a center core and an outer core, an inner cover layer andan outer cover layer, wherein the solid core is molded from a rubbercomposition comprising 100 parts by weight of a base rubber composed of(a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of atleast 60% and a 1,2 vinyl content of at most 2%, having a viscosity η at25° C. as a 5 wt % solution in toluene of up to 600 mPa·s, beingsynthesized using a rare-earth catalyst, in combination with (b) 0 to 80wt % of a diene rubber other than component (a), (c) 10 to 60 parts byweight of an unsaturated carboxylic acid or a metal salt thereof orboth, (d) 0.1 to 5 parts by weight of an organosulfur compound having 2to 4 sulfurs, (e) 5 to 80 parts by weight of an inorganic filler, and(f) 0.1 to 5 parts by weight of an organic peroxide; the center core hasa JIS-C hardness of 40 to 60 on its center and a JIS-C hardness of 55 to75 on its surface and the difference therebetween is at least 10, theouter core is harder than the surface hardness of the center core, thecross-sectional hardness of 1 mm outside from the border between thecenter core and the outer core is from 65 to 85 on a JIS-C hardness, asurface of the outer core has a JIS-C hardness of 75 to 95, the innercover layer has a Shore D hardness of 50 to 80, the outer cover layerhas a Shore D hardness of 35 to 60, and the outer cover layer has alower Shore D hardness than the inner cover layer.
 37. The golf ball ofclaim 36, wherein the polybutadiene (a) satisfies relationship:10B+5≦A≦10B+60, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) ofthe polybutadiene and B is the ratio Mw/Mn between the weight-averagemolecular weight Mw and the number-average molecular weight Mn of thepolybutadiene.
 38. The golf ball of claim 36, wherein the diene rubber(b) includes 30 to 100 wt % of a second polybutadiene which has acis-1,4 content of at least 60% and a 1,2 vinyl content of at most 5%,has a Mooney viscosity (ML₁₊₄ (100° C.)) of not more than 55, andsatisfies the relationship:η≦20A−550, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) of thesecond polybutadiene and η is the viscosity of the second polybutadiene,in mPa·s, at 25° C. as a 5 wt % solution in toluene.
 39. The golf ballof claim 38, wherein the second polybutadiene in component (b) issynthesized using a Group VIII catalyst.
 40. The golf ball of claim 36,wherein the center core has a diameter of 15 to 36 mm and the outer corehas a thickness of 1.5 to 10 mm, and the inner cover layer has athickness of 0.2 to 3.0 mm and the outer cover layer has a thickness of0.2 to 2.0 mm.
 41. A multi-piece solid golf ball comprising a solid coreconsisting of a center core and an outer core, an inner cover layer andan outer cover layer, wherein the solid core is molded from a rubbercomposition comprising 100 parts by weight of a base rubber composed of(a) 20 to 100 wt % of a polybutadiene having a cis-1,4 content of atleast 60% and a 1,2 vinyl content of at most 2%, having a viscosity η at25° C. as a 5 wt % solution in toluene of up to 600 mPa·s, beingsynthesized using a rare-earth catalyst, in combination with (b) 0 to 80wt % of a diene rubber other than component (a), (c) 10 to 60 parts byweight of an unsaturated carboxylic acid or a metal salt thereof orboth, (d) 0.1 to 5 parts by weight of an organosulfur compound, selectedfrom a group consisting of diphenylpolysulfides, dibenzylpolysulfides,dibenzoylpolysulfides, dibenzothiazoylpolysulfides anddithiobenzoylpolysulfides, (e) 5 to 80 parts by weight of an inorganicfiller, and (f) 0.1 to 5 parts by weight of an organic peroxide; thecenter core has a JIS-C hardness of 40 to 60 on its center and a JIS-Chardness of 55 to 75 on its surface and the difference therebetween isat least 10, the outer core is harder than the surface hardness of thecenter core, the cross-sectional hardness of 1 mm outside from theborder between the center core and the outer core is from 65 to 85 on aJIS-C hardness, a surface of the outer core has a JIS-C hardness of 75to 95, the inner cover layer has a Shore D hardness of 50 to 80, theouter cover layer has a Shore D hardness of 35 to 60, and the outercover layer has a lower Shore D hardness than the inner cover layer. 42.The golf ball of claim 41, wherein the polybutadiene (a) satisfiesrelationship: 10B+5≦A≦10B+60, wherein A is the Mooney viscosity (ML₁₊₄(100° C.)) of the polybutadiene and B is the ratio Mw/Mn between theweight-average molecular weight Mw and the number-average molecularweight Mn of the polybutadiene.
 43. The golf ball of claim 41, whereinthe diene rubber (b) includes 30 to 100 wt % of a second polybutadienewhich has a cis-1,4 content of at least 60% and a 1,2 vinyl content ofat most 5%, has a Mooney viscosity (ML₁₊₄ (100° C.)) of not more than55, and satisfies the relationship:η≦20A−550, wherein A is the Mooney viscosity (ML₁₊₄ (100° C.)) of thesecond polybutadiene and η is the viscosity of the second polybutadiene,in mPa·s, at 25° C. as a 5 wt % solution in toluene.
 44. The golf ballof claim 43, wherein the second polybutadiene in component (b) issynthesized using a Group VIII catalyst.
 45. The golf ball of claim 41,wherein the center core has a diameter of 15 to 36 mm and the outer corehas a thickness of 1.5 to 10 mm, and the inner cover layer has athickness of 0.2 to 3.0 mm and the outer cover layer has a thickness of0.2 to 2.0 mm.